The present invention relates to the production of inherently tacky, elastomeric pressure-sensitive adhesive microspheres useful in the production of removable and repositionable note paper, tape and label products.
The mid-1970's saw the introduction of removable and repositionable note papers and tapes which have found broad acceptance in the marketplace. The adhesives utilized in some of the products were infusible, inherently tacky, elastomeric microspheres prepared by an aqueous suspension polymerization process.
U.S. Pat. No. 3,620,988 to Cohen, incorporated herein by reference, teaches the synthesis of suspension beads (microspheres) from acrylic monomers in the presence of a water insoluble suspension stabilizer such as crosslinked copolymer of acrylic acid. The monomer composition indicates that these beads would likely have been tacky microspheres.
U.S. Pat. No. 3,691,140 to Silver, incorporated herein by reference, teaches the synthesis of infusible, tacky microspheres of alkyl acrylates in the presence of an ionic comonomer which is substantially oil insoluble and in the presence of an anionic emulsifier provided at a level above its critical micelle concentration (CMC). A wide variety of ionic comonomers are disclosed such as imides, salts of acrylic acid and maleic anhydride.
U.S. Pat. No. 3,912,581 to Fink, incorporated herein by reference, teaches the synthesis of suspension polymers of acrylic monomers in the presence of water soluble stabilizers or water-insoluble suspending agents. Water soluble stabilizers disclosed are partially hydrolyzed polyvinyl acetate or sodium salt of a copolymer of methacrylic acid and one of its higher alkyl acrylate esters. Aluminum hydroxide is used as a water-insoluble suspending agent. An anionic emulsifier is also used to further improve suspension stability.
U.S. Pat. No. 4,166,152 to Baker, et al, incorporated herein by reference, describes a method for making inherently tacky microspheres based on acrylic monomers in the presence of an ionic suspension stabilizer and an anionic emulsifier.
U.S. Pat. Nos. 4,495,318 and 4,598,112 to Howard, each incorporated herein by reference, teach a method of forming the tacky microspheres wherein nonionic or cationic emulsifiers are used in combination with an ionic suspension stabilizer.
In both Baker et al. and Howard, when the suspension stabilizer is polyacrylic acid, it is neutralized to a pH of 7 with ammonia to Convert it to ammonium salt. No mention is made of the need to regulate the pH during polymerization to obtain reproducible and stable tacky microspheres.
U.S. Pat. No. 4,786,696 to Bohnel, incorporated herein by reference, teaches a process for making tacky microspheres without the use of ionic comonomers taught by Silver or ionic suspension stabilizers taught by Baker et al or Howard.
U.S. Pat. No. 4,833,179 to Young et al, incorporated herein by reference, teaches a process for making suspension polymer beads in the presence of suspending agents which are water-soluble inorganic salts such as tribasic calcium phosphate, barium sulfate, magnesium carbonate and the like, in addition a modifier moiety such as polystyrene macromer, reactive zinc salt or hydrophobic silica and the like. The presence of the modifier moiety with an adsorbed layer of inorganic suspending agent stabilizes the microspheres against coagulation when stored at high solids content.
The presence of surfactants and stabilizers have been observed to have a deleterious effect on the adhesive properties of the microspheres.
It is therefore desirable to make the microspheres in the absence of stabilizers and by use of low levels of surfactants.
The above identified patents do not recognize the significance of pH control in the suspension polymerization of alkyl acrylate monomers in order to obtain stable microsphere suspensions. This has been found to be especially true when the microspheres are made in the absence of suspension stabilizers as taught by Bohnel as attempts to duplicate the process of Bohnel has led to failure.
In addition, and as will be shown, the stability of the microspheres as made by the process of Silver varies considerably depending on acidity of the suspension, either before or at conclusion of polymerization.